Layered, one-sided etched color selection electrode

ABSTRACT

A curved, layered color selection electrode for a color cathode ray tube, and a method of making same. The electrode has a curved, relatively thin metal aperture-defining layer susceptible of being etched by a first etchant, at least one intermediate layer bonded to the convex surface of the aperture-defining layer and a relatively thick substrate layer bonded to the convex surface of the intermediate layer. The intermediate layer is more easily etched by the first etchant than is the aperture-defining layer, while the substrate layer is susceptible of being etched by a second etchant to which the intermediate layer and the aperture-defining layer are relatively resistant. The electrode is etched through all layers from one side to form a pattern of apertures whose walls have a one-sided double etched profile, thereby providing less aperture undercutting than prior art onesided etched electrodes.

United States Patent 1m uu 3,909,656

Stachniak Sept. 30, 1975 LAYERED. ONE-SIDED ETCHED COLOR SELECTION ELECTRODE lriirzur Etuminer-Robert Sega] [75[ Inventor: Raymond M. Stachniak, Chicago. Artur/M? AWN! Fm" juhn lll.

I73] Assignee: Zenith Radio Corporation, Chicago. ABSTRACT lll. A curved. layered color selection electrode for a color cathode rav tube. and a method of makin Y same. The {22] Fllcd: 1974 electrode has a curved relatively thin metal aperture- [Zl] Applr No.1-166J02 defining layer susceptible of being etched by u first etehant, at least one intermediate layer bonded to the convex surface of the aperture-defining layer and a relatively thick substrate layer bonded to the convex g Field NIH/4H7 1 408 407 surface at the intermediate la er. The mtermedrate layer is more easily etched by the hrst etehant than is References cued the aperturedefining layer, while the substrate layer is V susceptible of being etched by a second etchant to I ED STA PATEN1 S which the intermediate layer and the aperture-defining 211 3.83 12/1 53 La l, 313/403 X layer are relatively resistant The electrode is etched 37331108 EH95? Bllmsid 3HUM" X through all layers from one side to form a pattern of m apertures whose walls have a one-sided double etched .J ianea.,,. i e... t i 1 x 3 777.2U3 lZ/|'-)73 Yumuda et all r r r r 3 3/40] $777306 12/1973 Armstrong .v 3|5/l69 R 4 ClaimsQ [4 Drawing Figures 1794.873 2/1974 Kaplan et al .v 3l3/4ll2 US. Patent Sept. 30,1975 Sheet 2 of4 3,909,656

US. Patent Sept. 30,1975 Sheet 3 of4 3,909,656

US. Patent Sept. 30,1975 Sheet4 Of4 3,909,656

LAYERED, ONE-SIDED ETCl-lED COLOR SELECTION ELECTRODE BACKGROUND OF THE INVENTION This invention relates generally to cathode ray tubes for color television and specifically to construction of improved color selection electrodes therefor.

Every commercial color television CRT (cathode ray tube) includes a color selection electrode which allows a selected pattern of electrons to impinge upon a corresponding pattern of light emitting phosphor elements on a CRT screen. A typical color selection electrode is of the shadow mask variety, consisting of a thin sheet of steel having a pattern of electron transmissive apertures etched therein. The apertures may be in the form of small round holes, vertically running slots, or other shapes.

Naturally, it has been desirable to etch the mask apertures as accurately as possible in order to provide each mask with a uniform aperture pattern, every aperture accurately positioned and having a predetermined size and shape.

The most straight-forward method of forming the mask apertures is to coat one side of the mask blank with an etchant-resistant coating in which there is a pattern of holes through which an etchant can be applied to the mask. The mask is then etched by spraying an etchant onto the portion of the mask surface which is uncovered by the holes in the etchant-resistant coating. This spray is normally continued until the etchant mills a hole through the mask. Since this method of forming mask apertures includes etching from only one side of the mask, it is referred to herein as one-sided etching.

Unfortunately, it has been found that it is difficult to control the size of the holes which are formed in a mask with one-sided etching. For example, variations in the thickness of a mask, which is normally made of 6 mil steel, coupled with a tendency of etching to vary in accordance with the grain structure of the mask metal, tends to introduce undesirably large variations in the size and shape of the mask apertures. A particularly undesirable characteristic of one-sided etching is that, while the etchant is creating an opening in the steel in the direction of the thickness of the mask, it is also eating away at or undercutting the metal of the mask laterally beneath the etchant-resistant coating which lies on the surface of the mask. This undercutting continues as long as the etchant is applied to the mask and is definitely undesirable from the standpoint of mask strength. The thicker the mask is, the longer the etchant must be applied to completely etch through the mask and the greater the amount of undercutting which occurs. Typically, an etchant will undercut or etch laterally about (1.5 mils for every l mil of through etching. Thus, for a 6 mil thick mask, undercutting may eat away up to 3 mils or more of the mask which is covered by the etchant-resistant coating.

in order to avoid the undesirably large amount of undercutting which is associated with one-sided etching. a two-sided method of etching masks has been developed. Such a method is disclosed and claimed, for ex ample, in US. Pat. No. 3,676,914, issued to Fiore and assigned to the assignee of this invention.

Briefly. two-sided etching includes applying an etchantresistant coating or resist to both sides ofa mask and then forming corresponding registered patterns of holes in both coatings. Etchant can then be applied to both sides of the mask so that etching will occur simultaneously from both sides towards the center of the mask. Since two-sided etching can etch through the mask faster than one-sided etching, less undercutting will occur and the resultant mask will be structurally stronger.

A major disadvantage of the two-sided etching approach is that the hole patterns in the resists on either side of the mask must be in accurate alignment with each other in order to produce mask apertures in the proper shape. If they are not in alignment, either no apertu re will be produced in the mask or the aperture will have a shape which is undesirable, depending on the degree of misalignment. But, even with this rather strict alignment requirement, two-sided etching has proven to be a commercially practical method of etching mask apertures. However, even in view of its present commercial success, two-sided etching still has at least one major drawback; namely, that its use is limited to etching masks blanks which are flat. That this limitation exists and that it can be a distinct disadvantage will become apparent in the discussion immediately below.

In the production of a CRT mask, it is customary to start with a flat blank piece of metal. Apertures are then etched through the metal according to the twosided etching process described above, for example. Sometime after the apertures have been etched, the mask is subjected to a process referred to as forming in which the etched blank is mechanically worked to give it a curved contour which corresponds to the contour of the CRT screen. By virtue of such working, the pattern of etched apertures in the mask usually becomes somewhat deformed. Such deformation is not critical in the conventional tube making approach wherein each mask is uniquely mated with a particular screen, since the irregularities in any given mask also appear as compensating irregularities in the phosphor pattern on the mating CRT screen. However, such irregularities created in the pattern of apertures are not uniform from mask to mask and thus rule out interchangability among masks, a goal of CRT production which could lead to processing economy and greater tube uniformity.

Even where the deformation of the aperture pattern is not critical, forming the mask after it has been etched sometimes causes a mask to be damaged since it has already been weakened by the etching process.

lt follows that it is very desirable to be able to etch a mask blank after it has been formed in order to avoid damaging the mask and to avoid the above-mentioned deformation of the aperture pattern which occurs when forming follows etching. However, it is extremely difficult to practice two-sided etching on curved, preformed mask blanks because of the requirement that, with a two-sided etch process, the resist hole patterns on opposite sides of the mask must be in accurate alignment. To deposit registered resist hole patterns on the convex and on the concave side of a formed mask blank is exceedingly difficult, due in large part to the severe requirements imposed on the resist exposure optics. A thorough treatment of this subject may be found in U.S. Pat. No. 3,676,914, Fiore, directed to systems and methods for making color CRTs having inter changeable masks.

The severe Uiluc.UtiilIng associated with one-sided etching would likewise seem to rule it out as a practical alternative to two-sided etching of curved, preformed masks. Thus, the etching of preformed masks would appear to be commercially unattractive unless a way could be found to apply a one-sided etching process to the masks in a way that would minimize mask undercutting and the attendant structural weakness of masks so etched.

It would be particularly attractive to minimize under cutting in one-sided etching of preformed slot masks which have a series of vertically running slats separated by openings or slots through which electrons pass on their way to the CRT screen. The slots are held together by horizontally running tiebars which, for the sake of maximized brightness and minimum moire pattern generation, should be as narrow as possible. Up until now, one-sided etching has been almost completely ruled out for slot masks since the resultant undercutting weakened the tie bars to the extent that they had to be undesirably wide in order to be of sufficient strength.

The recent introduction of color CRTs having wide deflection angles (1 and greater), has caused more attention to be paid to another mask problem, that of doming. As used herein, doming means the localized expansion of a portion of a CRT mask due to local heating of the mask by electron bombardment. Because the mask and screen are closer to the electron guns in wide angle deflection CRTs, the electrons generated by the gun strike the mask at higher energies and thereby impart to the mask more heat than in the conventional 90 deflection tubes.

When doming occurs, the aperture pattern in the area of the mask which domes no longer is in registry with the corresponding pattern of phosphors on the CRT screen. This misregistry between the mask apertures and the screen phosphors results in local degradation of color purity as seen by a television viewer.

Although the problem of doming is apparently unre lated to the problems described above in connection with mask etching, it is referred to at this point in order to lay a foundation for the disclosure to follow which offers solutions to both the doming and the etching problems.

OBJECTS OF THE INVENTION It is a general object of this invention to provide an improved mask for a color cathode ray tube and a method for making same.

It is a more specific object of this invention to provide a mask having apertures which are etched from only one side but which do not exhibit the severe uridercutting which has been characteristic of one-sided etched apertures.

It is another object of this invention to provide a cathode ray tube and a slot mask therefor whose slots are formed by a one-sided etching process and whose tie bars exhibit the strength normally associated with a two-sided etched mask.

It is another object of this invention to provide a method for etching a curved, preformed mask blank, which method avoids the undesirable registration prob lems associated with two-sided etching and which also avoids the severe undercutting normally associated with onesided etching.

It is another object of this invention to provide a mask which has the above-mentioned desirable characteristics, but which is also resistant to doming induced by localized mask heating.

PRIOR ART US. Pat. Nos. 2,663,821; 2,728,008; 2,97l,1l7; 3,398,309; 3,689,792; and 3,777,206.

BRIEF DESCRIPTION OF THE DRAWINGS The features of this invention which are believed to be novel are set forth with particularlity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in conjunction with the accompanying drawings in which:

FIGS. 1 and IA are schematic Sectional views of a prior art single-layered CRT mask whose apertures are etched from one side;

FIG. 2 is a schematic sectional view of a doublelayered prior art mask whose apertures are etched from one side;

FIGS. 3 and 3A are schematic sectional views of prior art masks whose apertures are etched from both sides of the electrode;

FIGS. 4 and 4A are sectional views ofa layered color selection electrode according to this invention;

FIG. 5 is a schematic view of a cathode ray tube having a slotted shadow mask;

FIG. 5A is an enlarged view of a portion of the shadow mask in the FIG. 5 tube;

FIG. 6 is a sectional view of a shadow mask illustrating the high degree of undercutting associated with prior art one-sided etching;

FIGS. 7 and 8 are sectional views of color selection electrodes in accordance with this invention;

FIG. 9 is a view of a portion of a slotted mask etched in accordance with the principles of this invention; and

FIG. 9A is an enlarged view of a portion of the FIG. 9 mask.

DESCRIPTION OF THE PREFERRED EMBODlMENT In order to place this invention in its proper context and for the sake of comparing it with related prior art, a brief discussion of that prior art will be given before the preferred embodiment of the invention is discussed.

Referring first to FIG. I, there is shown a portion of a shadow mask blank 10 which is to be etched from its top side only. The blank is covered on its top side with a layer of resist 12 in which an opening 14 has been formed. (The curvature of the mask shown in this and the following figures has been exaggerated in order to more clearly point out which side of the mask is being operated on.)

In forming apertures through the mask blank, etchant is normally sprayed onto the resist-covered side of the blank so that apertures can be milled through the blank in places where openings 14 expose the surface of the blank to the etchant. As pointed out above, when an aperture is etched through a mask blank, the etchant also tends to eat away laterally under the resist coating. The result is a mask aperture having tapered sides as depicted by the dashed lines 16 in FIG. I. The distance in FIG. I is the distance over which undercutting of the resist layer normally occurs.

Once the apertures have been etched through the blank, the resist coating is removed therefrom, leaving a mask with apertures 17 formed such as that shown in FIG. 1A. Such a mask has been structurally weakened because of the great amount of metal removed by the undercutting. Another drawback of such a one-sided etch is that the size of aperture 17 is difficult to control and, therefore, unpredictable. Since hole size, that is, the effective aperture opening, is somewhat dependent upon the grain structure of the mask metal, the thickness of the mask and the etchant and etchant application parameters, it is exceedingly difficult to maintain a uniform hole size from mask to mask. The dimension W of aperture l7 in FIG. 1A is particularly variable since it is dependent on the amount of undercutting.

In order to more precisely control the hole size of mask aperture formed by a one-sided etching process, a new mask has been developed which has a thin, aperture-defining layer and a relative thick substrate layer. A portion of such a mask 10a is shown in FIG. 2. The aperture-defining layer 19 is made of nickel, for exam ple, while the substrate layer 18 is made of steel. Such a mask is disclosed and claimed in US. Pat. No. 3,794,873, assigned to the assignee of this invention.

The way in which the hole size of apertures in the FIG. 2 mask is controlled may be stated briefly as follows. A resist is applied to the top surface of layer 19 and openings are formed in the resist through which a corresponding hole is to be etched through layer 19. An etchant is then sprayed onto the mask and a hole is thereby etched through layer 19. Since layer 19 is very thin, the etchant need be applied thereto for only a short time in order to completely etch through it. As a result of the small etching time required, there is very little undercutting of the resist and the aperture has a well defined, predictable hole size.

Next, a different etchant, one that attacks substrate layer 18 but to which layer 19 is relatively resistant, is then sprayed onto the mask. This second etchant now mils through the substrate layer and undercuts layer 19, as expected. Although this undercutting does tend to structurally weaken the mask, the effective hole size is now precisely controlled by the aperture-defining layer 19. The degree of undercutting beneath the aperturedefining layer may now vary from aperture to aperture or from mask to mask without affecting the size of the aperture which an electron beam sees.

One way which is employed to avoid the severe undercutting associated with one-sided etching is to etch a mask from both sides. See FIG. 3. in this case, mask 10!) has an aperture 20 which has been formed by etching from both sides of the mask towards the middle. Note that the amount of undercutting in the FIG. 3

, mask is approximately one-half that of the mask shown in FIG. IA which was etched from only one side. Of course, in order to practice two-sided etching, one must begin with a mask which has resist on both surfaces. The pattern of openings in one layer of resist must be in registry or alignment with the pattern of openings in the resist on the opposite side of the mask.

A commercial version of a mask made with a twosided etching process is shown in FIG. 3A. ln this case, aperture 22 of mask IOcdiffers from aperture 20 of the FIG. 3 mask in that it is formed by etching most of the way through the mask from the bottom in order to form a first large recess in the mask and then by etching from the top, or electron gun side of the mask, a short distance to complete the etch-through of the aperture. Since the size of the hole facing the electron gun defines the effective size of the aperture, and since very little etching was required to form that portion of the aperture, the effective size of that aperture has been fairly well controlled. Such a mask is structurally stronger than one etched from only one side and has found widespread acceptance in the TV industry.

The pros and cons of the various prior art masks having been pointed out, the discussion will now turn to the preferred embodiment of this invention.

FIG. 4 depicts a sectional view of a shadow mask 10d and FIG. 4A an enlarged portion thereof constructed in accordance with this invention. As shown, the mask includes a relatively thick substrate 24, an intermediate layer 26 bonded to the concave surface of the substrate layer, and a relatively thin aperture-defining layer 28 bonded to the concave surface of the intermediate layer. Apertures 30 consisting of holes 31, 33 and 35 have been etched through all three layers. The walls 31W, 33W and 35W of apertures 30 have a one-sided double-etched profile, the formation of which will be discussed below.

According to this invention, apertures 30 have been etched through the mask by applying etchant to only one side thereof, in the case shown in FIGS. 4 and 4A, from the concave side of the mask. In addition, the lat eral etching of the mask has been reduced as compared to the onc-sided etched apertures of FIGS. 1 and 2. This means that not only can the mask be etched after it has been formed (since resist need be applied to only one side of the mask) but it will also be much stronger structurally than prior art masks whose apertures were etched from only one side. The way in which such desirable results have been achieved will now be explained.

The material composition of aperture-defining layer 28 and intermediate layer 26 (FIG. 4A) are such that they are etchable by a first etchant, but with the intermediate layer 26 being more easily etched by that first etchant than aperture-defining layer 28. Preferably, the aperture-defining layer is at least ten times and normally twenty times more as resistant to the first etchant as the intermediate layer is. Substrate layer 24 is of a material, preferably steel, which is susceptible of being etched by a second etchant to which intermediate layer 26 and aperture-defining 28 are relatively resistant.

Should it be desirable to produce a mask having more than three layers, one may add one or more additional intermediate layers so long as each such layer is etchable by an etchant to which each preceeding layer is relatively resistant.

In accordance with this invention, the mask and its apertures are made by first forming a flat mask blank into a curved blank having a contour which corresponds generally to the contour of a CRT screen of the type with which it is to mate. An etchant-resistant stencil containing a pattern of etchant-transmissive openings is then photo-chemically formed on the concave side of the blank. After the stencil has hardened, the first etchant is applied to the concave side of the blank to etch through the aperture-defining layer and the intermediate layer a pattern of apertures which corresponds to the pattern of etchant-transmissive openings in the stencil. The second etchant, which readily at tacks the substrate layer but less readily the aperturedefining and intermediate layers, is applied to the concave side of the mask to etch through the substrate layer in the locations previously etched by the first etchant.

In one embodiment of this invention, intermediate layer 26 preferably is made of copper and aperturedefining layer 28 is made of nickel. in that embodiment, apertures 30 may be conveniently etched by subjecting the concave side of the mask to a spray of ferric chloride (4245 Baume) until the etchant eats through layers 28 and 26 so that the minimum diameter of hole 33 in layer 26 is greater than the diameter of hole 31 in layer 28. This will insure that hole 3] in layer 28 determines the effective hole size of the resultant aperture. Since layer 26 is more readily etched by ferric chloride than layer 28 is, the hole 33 in layer 26 will be completed before the hole 31 in layer 28 expands laterally a significant amount.

Next, a solution of ferric sulfate, 35-40% by weight, is sprayed onto the concave side of the mask in order to etch through the steel substrate layer. Since ferric sulfate does not readily etch either nickel or copper, the size of the holes previously etched therein remains substantially the same. The ferric sulfate spray should be continued until the smallest diameter of hole 35 formed in substrate layer 24 is larger than the diameter of hole 31 formed in aperture-defining layer 28.

The result is a three layered mask whose apertures exhibit a one-sided double-etched profile, that is, the profile as depicted by aperture walls 31W, 33W, 35W in FIG. 4 wherein the walls of the holes in the intermediate layer and in the substrate layer have the depicted taper which is characteristic of holes successively etched from the same side of the mask. Since layer 28 is relatively thin, the taper of wall 31W is insignificant and is not shown.

The one-sided doubleetehed profile should be contrasted with a typical two-sided etched profile as depicted in FIG. 3.

As pointed out above, a mask according to this invention preferably has its apertures etched after the mask has been formed to give it a curved contour. As has been explained above, such a mask has an aperture pat tern which is not deformed by the working process used to shape the mask. Combining the advantage of having a nondeformed pattern of mask apertures with the concept of using an aperturedefining layer on the mask to produce apertures having easily controlled hole sizes can result in masks whose aperture pattern is so uniform from mask to mask and whose hole sizes are so predictable that a truly interchangeable mask system is possible. ln such a system, each CRT panel would be screened from master masks and the pattern of apertures in each mask would likewise be determined by a corresponding mask master. Then, any mask could be mated with any given panel.

in the embodiment depicted in FIGS. 4 and 4A, aperture-defining layer 28 preferably has a thickness of approximately 0.25 to 0.5 mils while intermediate layer 26 and substrate layer 24 have a combined thickness of from about 4 to 8 mils, with a mask preferably having a total thickness of about 6 mils. However in the case where intermediate layer 26 is made of copper and substrate layer 24 is made of steel, the bimetal effect may cause undesirably large stresses in the mask when the mask is heated. in that case, reducing the thickness of intermediate layer 26 to 1.5 or 2.0 mils, and correspondingly increasing the thickness of substrate layer 24, is recommended.

It should be pointed out that the numbers above for the thickness of the various layers are exemplary. Different applications of this invention may require, for example, that the thickness of aperture-defining layer 28 (FIG. 4A) be increased to 1 mil. Such changes in the thickness of the various layers are not deemed to depart from the essence of this invention.

Aperture-defining layer 28 may also be made of a copper-nickel alloy, preferably having about 50% copper. Increasing the copper content of the alloy improves the quality of the apertures etched therein (e.g., smoother, more uniform holes) but also unfortunately tends to make it more easily etched by the same etchant which attacks the intermediate copper layer. A 50-50 combination of copper and nickel appears to be a reasonable compromise, with the aperture-defining layer having good etch qualities while still being sufficiently more etch resistant to ferric chloride than the intermediate copper layer. The percent of copper in the intermediate layer may, however, be more or less than 50%, depending on the requirements of a particular application.

According to another aspect of this invention, the three-layered mask depicted in FIG. 4 is an improvement over prior art masks in another respect. As has been pointed out above, modern color CRT s and especially those having wide deflection angles, tend to experience localized expansion doming) of a portion of the mask due to local heating of the mask by electron bombardment. lf intermediate layer 26 of FIG. 4 is made of a heat conductive metal, such as copper, it will conduct away from the mask heat generated by impinging electrons and will substantially reduce mask doming.

In another embodiment of this invention which is particularly useful when doming is not a primary consideration, aperture-defining layer 28 (FIG. 4A) is made of nickel, intermediate layer 26 is an iron-nickel alloy, and substrate layer 24 is made of steel. This embodiment meets the requirement of this invention set forth above that the intermediate layer be more easily etched by a first etchant (ferric chloride) than the aperture-defining layer, and the substrate layer be etchable by a second etchant (ferric sulfate) to which the intermediate layer and the substrate layer are relatively resistant.

The percentage of nickel in the iron-nickel intermediate layer should be less than about 36% to avoid any bi-metal effect which may occur when a high composition nickel layer is sandwiched between a nickel and a steel layer. It is expected that an alloy of about 20-25% nickel will present no difficulties with a bi-metal effect, but will give that layer the desired etching characteristics.

The preferred thicknesses of the various layers in this particular embodiment are: approximately 0.5 mils for the nickel aperture-defining layer; approximately 1.5 mils for the iron-nickel intermediate layer; and approximately 4 mils for the steel substrate layer.

A recommended procedure for etching such a mask is to first etch an opening through the aperture-defining layer and the intermediate layer with a solution of ferric chloride. Then etch through the steel substrate layer with a solution of ferric sulfate. This produces through apertures of approximately the right size and shape. The mask may then be given a controlled etch with ferric chloride in order to round out the holes in the aperture-defining layer. Finally, the mask may be subjected to a controlled etch with ferric sulfate in order to expand the openings in the steel substrate layer wherever needed. This final ferric sulfate etch is best accomplished with the use of a densitometer to monitor the size of the openings in the substrate layer as set forth and claimed in US. Pat. No. 3,702,227. issued to Lerher, and assigned to the assignee of this invention.

A very important application of this invention relates to its use in slot mask tubes, one of which is depicted schematically in FIGS. and 5A. Slot mask tube 34 includes a source of at least one electron beam which may take the form of electron guns 36. The screen of the tube is coated with a repeating pattern of red (R), blue (8), and green (G) vertical phosphor stripes which are scanned by the electron beam. A threelayered slot mask 38 is supported adjacent and essentially parallel to the phosphor coated screen. It has a curved, relatively thick substrate layer, an intermediate layer bonded to the concave surface of the steel sub strate layer, and a relatively thin slot-defining layer bonded to the concave surface of the intermediate layer, similar to the layered mask shown in FIGS. 4 and 4A. The mask has a series of vertically running slots 40 separated by horizontally running tie bars 42 and vertically running slats 44.

Prior art slot masks are made by etching a pattern of slots in a mask blank and then forming the apertured mask. Since the tie bars are relatively narrow, the forming process may occasionally break some of the tie bars. By applying the principles of this invention to slot masks, the breakage of tie bars during forming can be eliminated since the slots can be etched after the mask has been formed.

In order to illustrate certain limitations of one-sided etching. especially when applied to slot masks, a brief discussion of one-sided etching as applied to a nonlayered slot mask will be given.

One should recall that it is desirable to have slot mask tie bars of minimum width in order to minimize moire pattern generation. Referring now to FIG. 6, there is shown a cross-sectional view of a single layer slot mask in which the noted dimensions are in mils. The top surface of the mask is covered by a photo-resist layer 44 which acts as a stencil for the tie bar and has openings through which slots 46 have been etched. In this case, the longest dimension of slots 46 runs horizontally across the page and has been purposely drawn smaller than scale for purposes of clarity. When viewed from the direction shown, each photo-resist layer defines the desired tie bar width which separates slots 46.

In this case, both the thickness of mask and the desired tie bar width are 6 mils. Since the etchant undercuts the photoresist layer 44 laterally an amount equal to approximately one-half the distance to be etched vertically. the tie bar width for such a one-layer mask must be 6 mils at least. Even with a 6 mil wide tie bar stencil as shown. the metal 48 beneath photo-resist layer 44 which defines the tie bar has been nearly completely etched away. Such a mask is, of course, impractieal and has been depicted only to point out how undercutting can limit the minimum width of tie bars in a slot mask.

FIG. 7 is a cross-sectional view of a slot mask etched in accordance with this invention. Once again, the longest dimension of slots 46A has been drawn smaller than scale to clarify the drawing.

Photo-resist layers 44A define the minimum width of tie bars which can be etched in a multi-layered slot mask when two of the layers together provide substantially all of the thickness of the mask. In this case, the mask is approximately 6 mils thick and has tie bars of only 3 mils since the amount of undercutting has been reduced by one-half over that depicted in FIG. 6. In general, the amount of undercutting will equal approximately one-half the width of the layer being etched.

Layer 50 is an aperture-defining layer which, because of its thinness and the fact that it is more resistant to its etchant than intermediate layer 52, has very little undercutting. Intermediate layer 52 has a thickness of 3 mils and, in accordance with the assumed rate of lateral etching, has been undercut by 1.5 mils on each side. Substrate layer 54, also 3 mils thick, has been undercut by its etchant 1.5 mils on each side, too.

The dimensions of the tie bars and the thickness of the layers in both FIG. 6 and FIG. 7 were chosen to illustrate the minimum tie bar width which could be achieved in each embodiment. Since respective layers in each mask have been almost completely undercut, they are, of course, impractical.

A slot mask according to this invention which has more practical dimension is shown in FIG. 8. Layers 44B are the photo-resist layers which define the desired tie bar width between slots 46B. Layer 50B is the aperture-defining layer and 528 the intermediate layer. In this case, intermediate layer 52B is only 2 mils thick while the tie bar stencil is 3 mils wide. Note how the decreased thickness of layer 528 and the attendant decrease in undercutting thereof contribute to a stronger base for aperture-defining layer 50B.

Since the thickness of intermediate layer 528 was reduced, the thickness of substrate layer 54B has been increased correspondingly to maintain the overall thickness of the mask constant. As a result, substrate layer 548 has been completely eaten away under intermediate layer 528 in the locations where the tie bars are. That the complete undercutting of substrate 548 is not a fatal drawback of the FIG. 8 mask cannot be appreciated from that figure. A view of the substrate side of the mask is shown in FIGS. 9 and 9A. Aperturedefining layer 503 can be seen by looking into slots 46B. It is evident that the aperture-defining layer determines the effective opening of slots 46B. Intermediate layer 528 is visible in the areas surrounding each slot 46B. Substrate layer 54B covers the remainder of the bottom surface of the mask.

The slot mask depicted in FIGS. 9 and 9A has a nondeformed pattern of slots whose hole size can be closely controlled and which, therefore, lends itself to interchangeability with other masks. In addition, when intermediate layer 528 is made from heat conducting material, the mask is resistant to doming induced by localized mask heating.

While the invention has been described with specific masks having specific dimensions and materials, it is evident that many variations therein will be apparent to those skilled in the art in light of the disclosure above. Accordingly. it is intended to embrace all such variations which fall within the spirit and scope of this invention as defined by the appended claims.

I claim:

1. For use in a color cathode ray tube. a curved, layered mask having electron-transmissive apertures etched only from its concave side, said mask compris- 3. A mask as set lorth in claim 1 wherein said aperture-defining layer has a thickness of from approximately 0.25 to 0.5 mils and the thickness of the entire mask is within the range of from 4 to 8 mils.

a curved, relatively thin nickel aperture-defining 5 4, F use i a Color h d ray tube, a

layer suscptlble elched y a first f fi dimensionally curved, three-layered slot mask etched at least one \mermedlate layer Composed of an from only its concave side and having narrow but relanickel alloy bonded to the convex surface of the tively Strong tie bars comprising: aperture'definmg layer and bemg more 62ml) a curved, relatively thick steel substrate layer; f by the first etchant than the aperture' an intermediate layer composed of an iron alloy dgfitlmg layfm and bonded to the concave surface of the substrate a relatively thick steel substrate layer bonded to the layer and g .7 gl of z g g g g i? a relatively thin, nickel slot-defining layer bonded to f fi ig tfi z i :3 :2 the concave surface of the intermediate layer, the I I y material composition of the three layers being such defining layer are relatively resistant, all layers are that he o fini g ye and the nte mediate t hed thr u h, at selected locations, with a atfe r n of apei" tu res whose walls have a one-sided d oulayer are both etchable by a first chemcal etchant ble-etched profile as would be formed by etching slot'definfng layer bemg at ten from the concave side of the mask, first through the as res'stam to f first etchant as the aperture-defining layer and the intermediate layer med'ate layer and the Substrate: layer being with said first etchant and then through the subetchable y a sficond to wiflch both strate layer with said second etchant at the loca- 'q g layer l the mtel'medlme f y are tions previously etched by said first etchant. relatwely g y Teslstam. thereby perm t ing l ts 2. A mask as set forth in claim 1 h vi th layers to be etched through the three layers by successive wherein the aperture-defining layer is at least ten times application of said first and second etchants withas resistant to the first etchant as is the intermediate out severely undercutting the aperture walls.

yer. 

1. FOR USE IN A COLOR CATHODE RAY TUBE A CURVED, LAYERED MASK HAVING ELECTRON-TRANSMISSIVE APERTURES ETCHED ONLY FROM ITS CONCAVE SIDE, SAID MASK COMPRISING: A CURVED, RELATIVELY THIN NICKEL APERTURE-DEFINING LAYER SUSCEPTIBLE OF BEING ETCHED BY A FIRST ETCHMENT, AT LEAST ONE INTERMEDIATE LAYER COMPOSED OF AN IRON-NICKEL ALLOY BONDED TO THE CONVEX SURFACE OF THE APERTURE-DEFINING LAYER AND BEING MORE EASLY ETCHED BY THE FIRST ETCHANT THAN THE APERTURE-DEFINING LAYER AND A RELAVATIVELY THICK STEEL SUBSTRATE LAYER BONDED TO THE CONVEX SURFACE OF THE INTERMEDIATE LAYER AND SUSCEPTIBLE OF BEING ETCHED BY A SECOND ETCHANT TO WHICH THE IMTERMEDIATE LAYER AND THE APERTURE-DEFINING LAYER ARE RELATIVELY RESISTANT ALL LAYERS ARE ETCHED THROUGH, AT SELECTED LOCATIONS, WITH A PATTERN OF APERTURES WHOSE WALLS HAVE A ONE-SIDED DOUBLE-ETCHED PROFILE AS WOULD BE FORMED BY ETCHING FROM THE CONCAVE SID OF THE MASK FIRST THROUGH THE APERTURE-DEFINING LAYER AND THE INTERMEDIATE LAYER WITH SAID FIRST ETCHANT AND THEN THROUGH THE SUBSTRATE LAYER WITH SAID SECOND ETCHANT AT THE LOCATIONS PREVIOUSLY ETCHED BY SAID FIRST ETCHANT.
 2. A mask as set forth in claim 1 having three layers wherein the aperture-defining layer is at least ten times as resistant to the first etchant as is the intermediate layer.
 3. A mask as set forth in claim 1 wherein said aperture-defining layer has a thickness of from approximately 0.25 to 0.5 mils and the thickness of the entire mask is within the range of from 4 to 8 mils.
 4. For use in a color cathode ray tube, a two-dimensionally curved, three-layered slot mask etched from only its concave side and having narrow but relatively strong tie bars, comprising: a curved, relatively thick steel substrate layer; an intermediate layer composed of an iron alloy bonded to the concave surface of the substrate layer; and a relatively thin, nickel slot-defining layer bonded to the concave surface of the intermediate layer, the material composition of the three layers being such that the slot-defining layer and the intermediate layer are both etchable by a first chemical etchant, but with the slot-defining layer being at least ten times as resistant to said first etchant as the intermediate layer, and with the substrate layer being etchable by a second etchant to which both the slot-defining layer and the intermediate layer are relatively highly resistant, thereby permitting slots to be etched through the three layers by successive application of said first and second etchants without severely undercutting the aperture walls. 